Before-After Control-Impact field surveys and novel experimental approaches provide valuable insights for characterizing stream recovery from acid mine drainage

Mineral extraction has resulted in widespread stream impairment due to habitat degradation and water quality impacts from acid mine drainage (AMD). The North Fork of Clear Creek (NFCC), Colorado, USA was historically impaired by AMD from two major point-source inputs, with some stream segments devoid of aquatic life prior to remediation. In the summer of 2017, the North Clear Creek Water Treatment Plant (NCCWTP) began AMD water treatment. To predict and characterize the biological recovery of NFCC to improvements in water quality, we conducted stream mesocosm and field experiments, as well as biomonitoring of benthic communities using a Before-After Control-Impact (BACI) study design. The NFCC stream community responded rapidly to improved water quality. Benthic algal biomass increased at impacted sites and macroinvertebrate surveys showed significant increases in abundance, taxa richness, and emerging adult aquatic insects. However, the dominant taxa colonizing downstream segments of NFCC differed considerably from those predicted based on previous field and experimental results. We hypothesize that this discrepancy is the result of differences in metal exposure regimes observed between our field and mesocosm approaches (i.e., fluctuating vs stable), colonization attributes (i.e., open vs closed system), and spatiotemporal differences in metal sensitivity due to macroinvertebrate phenology. We expect continued biological recovery in NFCC, but habitat impairment and residual sources of metals will continue to impair aquatic life until those stressors abate. Applying a combination of controlled experimental and BACI field approaches to predict and evaluate AMD-remediation projects in the future will improve the ability to understand the physical, chemical, and biological mechanisms influencing stream recovery.

Acid-adapted microalgae exhibit phenotypic changes for their survival in acid mine drainage samples

Phenotypic plasticity or genetic adaptation in an organism provides phenotypic changes when exposed to the extreme environmental conditions. The resultant physiological and metabolic changes greatly enhance the organism's potential for its survival in such harsh environments. In the present novel approach, we tested the hypothesis whether acid-adapted microalgae, initially isolated from non-acidophilic environments, can survive and grow in acid-mine-drainage (AMD) samples. Two acid-adapted microalgal strains, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, were tested individually or in combination (co-culture) for phenotypic changes during their growth in samples collected from AMD. The acid-adapted microalgae in AMD exhibited a two-fold increase in growth when compared with those grown at pH 3.5 in BBM up to 48 h and then declined. Furthermore, oxidative stress triggered several alterations such as increased cell size, granularity, and enhanced lipid accumulation in AMD-grown microalgae. Especially, the apparent limitation of phosphate in AMD inhibited the uptake of copper and iron in the cultures. Interestingly, growth of the acid-adapted microalgae in AMD downregulated amino acid metabolic pathways as a survival mechanism. This study demonstrates for the first time that acid-adapted microalgae can survive under extreme environmental conditions as exist in AMD by effecting significant phenotypic changes.